The present invention relates generally to the field of machining and fabrication of small-scale components. It relates more particularly to the fields of micro- and meso-scale machining, such as milling and drilling.
It is desirable to produce three-dimensional structures on a micro-scale or meso-scale (that is, 10 micron to 10 mm) consistently and with high resolution. Producing such structures efficiently and with reduced required costs is also desirable.
Lithography-based micro-machining processes are limited to formation of two-dimensional and layered structures and function with a limited set of available materials. Lithography is also a slow process (for example, a processing time controlled by etch deposition rate is usually measured in μm/min), and uses expensive equipment (for example, a typical microfabrication lab has millions of dollars in equipment).
Micro- and meso-scale milling and drilling are a subset of mechanical micro- and meso-scale machining processes. These are similar to traditional milling and drilling in that a rotating tool is used to remove material from a workpiece through chip formation. However, micro- and meso-scale milling and drilling use much smaller tools, e.g., ranging from 10-1000 μm in diameter, and incorporate various differences due to the scale of the process.
Micro- and meso-scale milling and drilling may be used to make three-dimensional geometries in a wide variety of materials with a flexible process. The process is suitable for a wide variety of materials including, but not limited to metals, plastics, composites, and glass. Micro- and meso-scale milling and drilling generally is more flexible in application than, for example, lithography-based microfabrication processes, because it is a single-point process with high material removal rates (several mm3/min), and it is driven by mechanical processes rather than chemical reactions. Parts created with micro- and meso-scale milling and drilling may be quite small (for example, less than 25 mm×25 mm×25 mm).
However, existing machine tools prior to the present invention that have the accuracy needed for micro- and meso-scale milling and drilling are relatively large (for example, table-sized). Additionally, such tools are expensive to manufacture (for example, costs may exceed $100,000).
Various designs for miniature micro- and meso-scale milling testbeds are known in the art. Such designs typically incorporate off-the-shelf components. Challenges exist that are associated with applying existing technology and design principles to miniature machine tool design.
An earlier micro-milling testbed described in Vogler, Liu, Kapoor, DeVor, Development of Meso-Scale Machine tool (mMT) Systems, Transactions of the North American Manufacturing Research Institution of SME (NAMRI), pp. 653-661, 2002, has a three-axis CNC machining capability. This testbed, built using off-the-shelf components, has an overall size of 125×180×300 mm, and a working volume of 25 ×15×25 mm. It has very high (5 g) acceleration capability and uses moving coil actuator stages. Also, the testbed is capable of measuring cutting force using a triaxial force sensor. However, this testbed provides limited machine stiffness, difficult workpiece mounting and interchange, difficult spindle interchange, unprotected moving parts, position-dependent counterbalancing force, and excessive tool runout.
Another machine, described in Werkmeister, Slocum, Design and Fabrication of the MesoMill: A Five-Axis Milling Machine for Meso-Scaled Parts, Proceedings of: Machining and Processes for Microscale and Meso-scale Fabrication, Metrology and Assembly, ASPE 2003 Winter Topical Meeting, has a design to create a miniature micro-milling machine with micron-level accuracy. This machine uses ballscrew splines, which are shafts with both helical and axial grooves (used to provide rotary and axial motion with one shaft) for its motion platform. The spindle used is a larger printed circuit board drilling spindle (120,000 rpm max) that feeds axially in addition to rotating. The machine has an overall size of 500×300×500 mm.
Still another machine, developed by Georgia Tech University, includes a movement platform of fiber-optic positioning stages. These stages have 5-axis capability, but have only a 4 mm/min maximum speed, which in particular applications is too limiting to achieve a good material removal rate, or to achieve the minimum feed-per-tooth for chip removal on each flute pass.
Yet another machine, described in R. Subramanian, K. F. Ehmann, Development of a Meso-Scale Machine Tool (mMT) for Micro-Machining, Japan-USA Symposium on Flexible Automation, Hiroshima, Japan, Jul. 14-19, 2002, has a small size (90×60×60 mm), but provides a number of drawbacks. For example, the machine uses piezoelectric stepper actuators that are sensitive to the applied cutting force. This sensitivity causes undesired velocity variations. Also, these off-the-shelf actuators have poor rigidity. Further, to maintain a small overall size, a dental spindle is used. Still further, the spindle lacks precision bearings, resulting in undesirable runout characteristics.